Project Summary: Our goal is to determine the molecular mechanisms that control contraction of antigenspecific CD8+ T cell responses following acute and chronic viral infections. At the peak of the response, effector CD8+ T cells contain low levels of Bcl-2 and are susceptible to Bim-mediated apoptosis. Activated CD8+ T cells are also susceptible to apoptosis through death receptors such as Fas, Tumor Necrosis Factor Receptor I (TNFRI) or Tumor necrosis factor Related Apoptosis Ligand-Receptor (TRAIL-R). Single mutation of either Bim or any one death receptor results in delayed contraction of antigen-specific CD8+ T cells following acute lymphocytic choriomeningitis virus (LCMV) infection but cell numbers eventually decline. However, loss of both Bim and Fas, a death receptor, results in a lymph node-specific block in contraction following acute LCMV infection. The specific hypothesis that is being addressed in this proposal is that the contraction of antigen-specific CD8+ T cells is controlled by both extrinsic and intrinsic death pathways. Specific Aim 1 will determine the mechanism by which Fas aids Bim in the contraction of antigen-specific CD8+ T cells in the lymph nodes following acute LCMV infection. First, we will determine if the defects in lymph node contraction are T cell autonomous by creating Bim-/-Faslpr/lpr P14 TCR transgenic mice. T cells from these mice will be adoptively transferred into wildtype mice and contraction will be measured. The cells that express Fas Ligand in the lymph node during contraction will be determined by flow cytometry and immunohistochemistry. The importance of expression will be determined by adoptive transfer of mutant P14 transgenic T cells into mice which selectively express functional Fas Ligand. In Specific Aim 2, we will determine the contribution of death receptor and Bim-mediated apoptosis in regulating contraction. This will be accomplished by creating Bimdeficient mice that inducibly express a dominantly interfering FADDdd protein, which blocks death receptor signaling, in their T cells. The sensitivity to apoptosis following exposure to extrinsic and intrinsic deathinducing stimuli will be measured in na[unreadable]ve and activated CD8+ T cells from Bim-/-rtTA-FADDdd in vitro. To determine the extent to which loss of all death receptor and Bim-induced apoptosis affects responses in other organs besides the lymph nodes, double mutant mice will be infected with strains of LCMV that induce acute and chronic infection and contraction of antigen-specific CD8+ T cells will be measured in vivo.